This disclosure relates generally to diagnostic imaging and, more particularly, to temperature control in a detector for a computed tomography (CT) gantry.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are transmitted to the data processing system via A/D ASICs for image reconstruction. Imaging data may be obtained using x-rays that are generated at a single polychromatic energy. However, some systems may obtain multi-energy images that provide additional information for generating images.
Components within the detector, to include the scintillator, the photodiodes, and the A/D ASIC, are temperature sensitive and are typically calibrated during system calibration. Thus, it is desirable to perform system calibration at the operating temperature of the system to avoid image quality issues that may arise if the temperature in these components drifts during system use. It is also therefore desirable to maintain the environment within the gantry at a controlled temperature to minimize thermal drift during use.
CT gantries are therefore typically air cooled, but may be operated in rooms that range in temperature from 18° C. to 26° C. Usage of the CT system can widely vary as well, resulting in the gantry air ranging in temperature from 18° C. to greater than 35° C. Thus there are widely varying temperatures that may be experienced within the gantry (caused by heavy or light usage of the x-ray tube and other components), and the ability to design systems that cool and control the inner gantry temperature may be further compounded because of the widely varying ambient temperatures that may be experienced.
To provide generally constant component temperature operation, some CT detector systems include a heater that is attached to the detector. A sensor on the detector and/or its components thus may be used as input to a heater controller, enabling a generally uniform temperature of the components to be achieved during calibration and during system operation. That is, the detector can be heated above the maximum gantry inner air temperature and, in conjunction with the gantry ambient airflow, detector component temperature uniformity can generally be obtained.
However, in recent years the A/D ASICs (which are a heat source) and other electronic components (i.e., on a DAS) have been moved closer to the photodiodes in an effort to improve signal-noise ratio in CT detectors. Although the signal-noise ratio may be improved, moving the heat source(s) closer to the detector components can also compromise the ability to globally and uniformly cool the detector within the gantry using the general environment within the gantry. As such, some system designs include a heat sink material that is placed in thermal contact with the ASICs and other heat sources on detector modules, and the heat sink materials are convectively cooled. That is, a CT system may include numerous detector modules, each of which includes a DAS card that is positioned in close thermal proximity to the photodiode array. The DAS card includes the ASIC(s) which are directly cooled via the heat sink and the convective air blown over them. Typically as well, the individual DAS/heat sink modules may be placed within a plenum through which air is blown, using fans that are directly coupled to the plenum. Heat generated in the DAS components is thereby convectively cooled, and heat transfer to the thermally sensitive components is thus controlled.
The plenum for cooling numerous DAS cards may include a plurality of fans, five in one example. Each fan therefore may provide convective flow for its own “zone” that may include 10-12 DAS cards, as one example. Further, thermal gradients may also develop between zones, each of which may experience very different thermal conditions due to geometric effects within the gantry. That is, the leading zone in the rotational direction may experience convective airflow fed to it fan, while the central or trailing zones may experience different amounts of available airflow.
Thus, given the widely varying ambient conditions, system operating conditions, and the geometric effects, fan speed for each zone is separately controlled. Fan speed is therefore cycled through a wide range of operating speeds during system use. However, individual zone control of the fans can cause crosstalk between zones within the plenum, and air blown in one zone can affect airflow in neighboring zone(s). Fan cycling can also lead to early life failure of the fan, leading to costly repairs. Fan cycling can also cause an increase in acoustic noise as the bearing in the fan ages. In fact, fan cycling itself can appear as a nuisance to a system user who, hearing the fan cycling, may suspect imminent failure of the fan or be simply annoyed by the cycling.
Therefore, it would be desirable to have a method and apparatus to improve thermal performance within a CT gantry.